Modern communication systems and those which include data transmission capability over telephone lines and the like have resulted in the design and development of modems capable of operating at high frequency rates to accommodate data and further accepting or transmitting both data and voice. A variable rate modem is employed to accommodate both data and voice information where the data is transmitted and received at different bit rates such as 2400 bits per second (bps) or 4800 bps in either the transmission or reception modes.
In utilizing such systems, as for example in conjunction with telephone lines, the quality of the line often limits the maximum data rate available. When voice is involved, the quality of the telephone line is not as significant as for example when data is employed. However, full duplex modems of variable operation are highly desirable. Thus there are modems that are available which achieve full duplex operation for both voice and data transmission and for other purposes as well. Echo cancellation techniques are also employed to improve the quality of the telephone link or transmission path utilized for full duplex transmission of voice and data. Furthermore, through the use of echo cancellation, telephone lines or satellite links of adequate quality for voice may often be successfully employed for the full duplex transmission and reception of data due to echo canceler operation.
A typical modem structure which is indicative of prior art techniques and which employs echo cancellation has been described in U.S. Pat. No. 5,005,168 entitled "MULTI-RATE WIRE LINE MODEM APPARATUS" issued on Apr. 2, 1991 to Peter Cummiskey et al. and assigned to ITT Corporation, the assignee herein. That patent describes a multi-rate wire line modem apparatus operable at either of two rates in either the transmission or reception modes and which provides full duplex operation and echo cancellation for both voice and data.
As the accuracy of data transmission increases, echo cancellation becomes more of a substantial problem and there is therefore a need to provide improved echo cancellation which is capable of removing both local echoes and remote echoes.
Echoes in telephone systems are due to mismatched impedances at 4 wire to/from two wire interfaces known as hybrids. The magnitude of the echo that a talker hears will depend on the echo path loss which is the sum of the return loss at the distant hybrid and the round trip loss in the circuit. Echo suppression or cancellation is basically a concern in telephony. Essentially, echo cancellation is an application of adaptive filtering technology to the control of echoes in the telephone network. As one understands, the telephone network generates echoes at points near the ends of a connection. Echo cancellation is a means which is used to combat the echo for both speech and data transmission. The requirements for speech and data are quite different so that the techniques employed for both are different. When voice is passed in digital form via modem apparatus, the modem apparatus can operate with either voice or data, and a single echo cancellation scheme operates for both voice and data.
In many telephone systems there are two wire segments on the ends of the telephone systems which are manifested in subscriber loops or a portion of a local network in which both directions of transmissions are carried on a single wire pair. The center of the connection is four wire in which the two directions of transmission are segregated on physically different facilities. This is necessary where it is desired to insert carrier terminals, amplifiers, or digital switches.
There is a potential feedback problem around the four wire portion of the connection and without sufficient loss in the path there is degradation of the transmission or in extreme cases oscillation or "singing" occurs. The hybrid is a device which provides a large loss around the loop thereby limiting this impairment. At the same time the hybrid must not insert significant loss in the two talker speech paths. In order that the path not have a large attenuation it is necessary for the hybrid not to have an appreciable attenuation between the two-wire and four-wire port. Talker echo results in the talker hearing a delayed version of his or her own speech while in listener echo the listener hears a delayed version of the talker's speech. Both these echo mechanisms are mitigated if the echo has significant loss between its two four-wire ports. For short delays the talker echo represents an insignificant impairment if the echo attenuation is reasonable. For longer delays, talker echo represents a serious impairment unless the echo is highly attenuated. However, such delays are of greater concern in data transmission, especially at variable rates.
With increasing round trip delay, the objective effect of echo becomes more annoying. The introduction of satellite transmission systems which have found trip delays of 500 milliseconds or more have introduced a significant new source of large transmission delay. Satellite transmission systems require echo control subsystems and these subsystems employ echo cancellation. Echo cancelers provide sophisticated forms of echo control which can effectively eliminate echo as an impairment, even on very long delay channels.
There are many different techniques which are available in the prior art and which implement echo cancellation. See, for example, a text entitled Advanced Digital Communications, by K. Feher, Editor, published by Prentice Hall Inc. (1987), a division of Simon & Schuster, New Jersey. Reference is made to Chapter 4 entitled "Echo Cancellation in Speech and Data Transmission" by Dr. David G. Messerschmitt. The use of echo cancellation including non-linear echo cancellation is fairly well understood. However, in order to provide optimum echo cancellation for data transmission it has been determined that one must consider echoes which result from local echo path disturbances as well as echoes which result from remote paths disturbances.
As indicated, modems operate in conjunction with telephone communication systems which also may interface with satellite communication links or trunks. Thus there are multiple sources of echo disturbances including the above-noted hybrid which is a major source of an echo problem and which results in local echo generation. Other sources of echo include frequency shift which may occur by the use of different oscillators or timing controls in the system as well as Doppler shifts which may occur due to different satellite positions or other causes as well. Local echoes are usually echoes which are up to 15 milliseconds in delay time; if the delay is greater than 15 milliseconds the echo is usually a remote echo caused by a remote disturbance.
Based on prior art techniques where delays of more than 45 milliseconds are encountered, echo suppressors were employed. An echo suppressor is a voice operated device which, while one subscriber is talking, inserts as much as 50 db loss in the opposite direction of transmission which is the path over which the echo would return. Such devices effectively suppress echoes but they introduce their own transmission impairments by clipping the beginning and ending of words. Another more serious problem occurs on a multi-link connection where two or more circuits equipped with echo suppressors must be switched together. In these systems it is possible for each subscriber to talk simultaneously and gain control over the echo suppressor nearest him. This causes lock out and all types of problems and such techniques were totally not suitable for data transmission.
A typical echo canceler generates a replica of the echo by applying a reference signal to a transversal filter (tapped delay line). The transversal filter coefficiencies are caused to adapt to the echo transfer function. The transversal filter for echo cancellation is well known. However to accommodate higher digital data rates and to provide greater communication system reliability, improved echo cancellation apparatus and techniques are necessary.
It is an object of the present invention to provide both remote and local echo cancellation adapted to operate with a multi-rate wire line modem apparatus, which modem is operable at either of two rates in either transmission or reception modes while providing full duplex operation. Such a system, including echo cancellation, as indicated has been described in U.S. Pat. No. 5,005,168.
It is a further object of the present invention to provide a remote and a local echo canceler which are useful for operation with a full duplex modem. The echo canceler operates to cancel echoes with delays in excess of one second which is indicative of two satellite hops with frequency offsets up to + or -8 Hz. The echo canceling apparatus to be described operates to cancel both remote and local echoes and can provide an echo compensating signal within a reasonably short period of time as compared to the round trip delay.